<h2>Problem 161</h2>
<div style="color:#666;font-size:80%;">21 September 2007</div><br />
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<p>A triomino is a shape consisting of three squares joined via the edges.
There are two basic forms:</p>

<p style="text-align:center;"><img src="http://projecteuler.net/project/images/p_161_trio1.gif" alt="" /></p>

<p>If all possible orientations are taken into account there are six:</p>

<p style="text-align:center;"><img src="http://projecteuler.net/project/images/p_161_trio3.gif" alt="" /></p>

<p>Any n by m grid for which nxm is divisible by 3 can be tiled with triominoes.<br />
If we consider tilings that can be obtained by reflection or rotation from another tiling as different there are 41 ways a 2 by 9 grid can be  tiled with triominoes:</p>

<p style="text-align:center;"><img src="http://projecteuler.net/project/images/p_161_k9.gif" alt="" /></p>

<p>In how many ways can a 9 by 12 grid be tiled in this way by triominoes?</p>
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